Base metals must be protected from corrosion. This applies to all usual structural metals such as iron, steel, zinc, titanium, aluminum, magnesium, or alloys thereof. These metals are usually equipped with one or more inorganic and/or organic coatings. In addition to corrosion protection, desirable aesthetic effects are also achieved in this context. A variety of coating types and coating methods are known and widespread in the existing art for this.
For example, it is usual to subject metal surfaces (cleaned, if necessary) to a so-called conversion treatment. In this, a coating forms into which ions of the metal surface are incorporated. Examples of this are chromating, layer-forming or non-layer-forming phosphating, or treatment with an acidic aqueous solution of complex fluorides of at least one of the elements B, Si, Ti, and/or Zr. Such conversion solutions can additionally contain organic polymers.
For a mild or merely temporary corrosion stress it may be sufficient to limit oneself to such a conversion treatment. As a rule, however, the conversion-treated metal surfaces are covered with one or more organic coatings of differing thicknesses. These organic coatings generally contain (crosslinked) organic polymers. With a suitably selected organic coating agent, however, a conversion treatment can also be omitted. This means that an organic coating agent based on (by preference, crosslinked or crosslinking) polymers can also be applied directly onto a bare metal surface. Different types of organic coating agents are known for this purpose. Crosslinking of the organic polymers is generally accomplished by way of one or more of the following reaction types: polymerization of compounds having multiple carbon-carbon bonds, formation of urethane bonds by reaction of isocyanates, ring-opening reaction of epoxies, formation of polyesters.
Crosslinking reactions of this kind can occur on the coated sheet upon, for example, thermally or radiation-chemically induced curing of the coating. It is also possible, however, to use coating agents which already contain crosslinked organic polymers that are dissolved or dispersed in a liquid medium. The final coating is created by so-called “drying” upon evaporation of the liquid medium.
The organic coating agents and coatings formed therewith can contain, in addition to the organic polymers, further components that improve the chemical and physical properties of the coating. For example, inorganic and/or organic pigments are often used for coloring, to adjust tribological properties, and/or for improved corrosion protection. One particular class of pigments is the so-called “conductivity pigments.” These impart to the coating sufficient electrical conductivity that the coated metal sheet can be electrically welded and/or coated with an electrocoating paint. Examples of such conductivity pigments are: powdered elemental metals such as, for example, iron, zinc, aluminum, nickel, manganese, magnesium, or alloys thereof; metal phosphides; metal sulfides; metal oxides; graphite; and carbon black. Coating agents of this kind, with which conductive organic coatings can be constituted on metal substrates, are described below.
Also known are so-called “self-depositing” coating agents. Examples of these are described in the documents WO 97/07163, U.S. Pat. No. 6,312,820, WO 03/026888, WO 03/042275, and the literature cited therein. The process of self-deposition (also referred to as autodeposition) is based on the fact that by the action of an acid in the coating agent, divalent or polyvalent metal ions are dissolved out of the metal surface to be coated. The dissolved metal ions react with negatively charged groups of organic polymers suspended in the coating agent. As a result, the suspension of the organic polymers is destabilized, and the suspended polymer particles coagulate and precipitate as a film on the metal surface. This process stops automatically when the metal surface is completely covered with the polymer film, so that no further acid attack on the metal surface can occur. The polymer layer thus produced on the metal surface is baked and cured in a subsequent step. This produces layers that generally have a thickness in the range from 5 to 25 μm.
The formation of an autophoretically deposited coating thus requires that divalent or polyvalent metal ions, which destabilize the polymer emulsion, can be dissolved out of the substrate. This is usually accomplished by allowing the self-depositing coating agent to act on a bare metal surface. An alternative to this, which results in a double coating, is described in WO 96/02384. According to this document, a metal surface is firstly covered with a first coating that contains a powdered metal. In a second step, a second, self-depositing coating is deposited onto this first layer. This is evidently possible because, upon action of the acid contained in the second coating agent, a sufficient quantity of divalent or polyvalent metal ions is dissolved out of the powdered metal in the first coating to allow the self-depositing coating agent to deposit as a second coating. For concrete examples of first and second coating agents for the formation of this double coating, reference is made to the aforesaid document.